EML

An example excerpt of an ISO document showing how this is used is shown here for reference:

<gmd:descriptiveKeywords>
    <gmd:MD_Keywords>
        <gmd:keyword xsi:type="PT_FreeText_PropertyType">
            <gco:CharacterString>NESTS</gco:CharacterString>
            <PT_FreeText>
                <textGroup>
                    <LocalisedCharacterString locale="#locale_fra_CAN">NIDS</LocalisedCharacterString>
                </textGroup>
            </PT_FreeText>
        </gmd:keyword>
        <gmd:type>
            <gmd:MD_KeywordTypeCode codeList="http://nap.geogratis.gc.ca/metadata/register/napMetadataRegister.xml#IC_110" codeListValue="RI_528">theme</gmd:MD_KeywordTypeCode>
        </gmd:type>
        <gmd:thesaurusName>
            <gmd:CI_Citation>
                <gmd:title>
                    <gco:CharacterString>
                        Ontario Ministry of Natural Resources (OMNR) Thesaurus
                    </gco:CharacterString>
                </gmd:title>
                <gmd:date>
                    <gmd:CI_Date>
                        <gmd:date>
                            <gco:Date>1972-06-22</gco:Date>
                        </gmd:date>
                        <gmd:dateType>
                            <gmd:CI_DateTypeCode codeList="http://nap.geogratis.gc.ca/metadata/register/napMetadataRegister.xml#IC_87" codeListValue="RI_366">creation</gmd:CI_DateTypeCode>
                        </gmd:dateType>
                    </gmd:CI_Date>
                </gmd:date>
                <gmd:date>
                    <gmd:CI_Date>
                        <gmd:date>
                            <gco:Date>2006-07-16</gco:Date>
                        </gmd:date>
                        <gmd:dateType>
                            <gmd:CI_DateTypeCode codeList="http://nap.geogratis.gc.ca/metadata/register/napMetadataRegister.xml#IC_87" codeListValue="RI_368">revision</gmd:CI_DateTypeCode>
                        </gmd:dateType>
                    </gmd:CI_Date>
                </gmd:date>
                <gmd:edition>
                    <gco:CharacterString>
                        Originally created to catalogue documents stored in the OMNR library and since modified
                    </gco:CharacterString>
                </gmd:edition>
                <gmd:citedResponsibleParty>
                    <gmd:CI_ResponsibleParty>
                        <gmd:organisationName>
                            <gco:CharacterString>Ministry of Natural Resources</gco:CharacterString>
                        </gmd:organisationName>
                        <gmd:role>
                            <gmd:CI_RoleCode codeList="http://nap.geogratis.gc.ca/metadata/register/napMetadataRegister.xml#IC_90" codeListValue="RI_414">pointOfContact</gmd:CI_RoleCode>
                        </gmd:role>
                    </gmd:CI_ResponsibleParty>
                </gmd:citedResponsibleParty>
            </gmd:CI_Citation>
        </gmd:thesaurusName>
    </gmd:MD_Keywords>
</gmd:descriptiveKeywords>

Two questions:

1) Looking at this example more closely, rather than just a date, I wonder if it would make sense to add an optional 'citation' field for the thesaurus, which would closely mirror the CI_Citation provided in ISO.

2) In general we prefer adding optional fields as they allow for easy forward transformations from older EML versions to new. However, if this information is optional, then the conversion to ISO won't be valid when the field is omitted. So, which is more important, backwards compatibility with EML, or ISO validity?